#include<iostream>

using namespace std;

int blocks(int x) {
	int ans = 2;
	if (x == 1) {
		return ans;
	}
	for (int i = 2; i <= x; i++) {
		ans += i;
	}
	return ans;
}


int main(void) {
	int n;
	cin >> n;
	cout << blocks(n) << endl;
	return 0;
}
